Superradiant laser holds bright promise

BOULDER, Colo. – A new “superradiant” laser that traps 1 million rubidium atoms into a 2-cm space between two mirrors produces a deep-red laser beam that could boost the performance of the most advanced atomic clocks, communications and navigation systems, and space-based astronomical instruments.

Scientists at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, developed the prototype, which has the potential to be 100 to 1000 times more stable than the best conventional visible lasers. The research appeared in the April 5 issue of Nature (doi: 10.1038/nature10920).

“The laser we built is not particularly stable or narrow in frequency,” physicist James Thompson of JILA/NIST told Photonics Spectra. “Mainly, our system experimentally demonstrates key physics that might allow for future lasers that would be orders of magnitude more narrow in frequency than the best lasers of today.”

JILA’s superradiant laser traps 1 million rubidium atoms in a space of about 2 cm between two mirrors. The atoms synchronize their internal oscillations to emit laser light. Courtesy of Burrus/NIST.
The device synchronizes the rubidium atoms with an engineering technique called “phase arrays,” in which electromagnetic waves from a large group of identical antennas are carefully synchronized to build a combined wave with special useful features that are not otherwise possible. The scientists cooled the rubidium atoms to 20 µK with a laser and levitated them using a one-dimensional standing wave of light between two finesse mirrors, Thompson said.

One set of 780-nm lasers was used to optically pump the atoms into a particular quantum state that is very stable if nothing else is done. When a second laser was introduced, the atoms decayed from this stable state to a lower state with the emission of photons into a mode of the optical cavity formed by the mirrors.

“The key is that the photons escape very quickly from the mirrors before they have a chance to act back on the atoms too much,” he said. “If they stick around for too long, vibrations of the mirrors can cause the frequency of the laser to become smeared out – the same limiting factor on the most frequency narrow lasers that can be built.”

Because the atoms are constantly energized and emit synchronized photons, on the average very few – less than one photon, in fact – stick around between the mirrors.

This average, calculated by the scientists based indirectly on the laser beam’s output power, is enough to maintain an oscillating electric field to sustain the atom’s synchronized behavior. Almost every photon escapes before it has a chance to bounce around the mirrors and disrupt the synchronized atoms, which in standard lasers causes laser frequency to wobble.

“The idea of operating at less than one photon was to really hammer home the idea that one can build a laser in which nearly all of the phase information is stored within the atoms (or gain medium),” Thompson said. “When this happens, the lasing frequency becomes highly independent of the frequency of the optical cavity. This might be key for reducing the impact of fundamental thermal mirror noise on the world’s most narrow frequency lasers at JILA, NIST and other places around the world.”

Just as important, he added, is the fact that such lasers could be moved out of the vibration-controlled laboratory and into real-world applications.

“This was just the first step and is really a physics model of what you would really like to build in a system such as Jun Ye’s strontium atomic clock system here at JILA,” he said. “Such a laser might improve the stability and accuracy of the best atomic clocks by several orders of magnitude.”

Superradiant lasers also may enable precise measurements at very long distances.

“For instance, a millihertz linewidth laser would have a coherence length of order the Earth-sun separation, while the best lasers we have now only have coherence lengths of order the Earth to moon separation,” Thompson said. “One can dream of looking for Einstein’s gravity waves or even using the long coherence length to synchronize distant optical telescopes like Hubble in order to build telescopes with unprecedented angular resolution, such as might be helpful in searching for planets.”

For the superradiant laser design to reach its full stability potential and to be of practical use, Thompson stresses that it will need to be re-created using different atoms, such as strontium or ytterbium, which both have very long-lived excited optical states that are better suited for advanced atomic clocks.

Next, the scientists will explore whether it is possible to build hybrid passive and active optical atomic clocks that are highly adaptable to changing vibration environments while still maintaining accuracy.

“We also would like to further understand the intrinsic stability properties of these lasers,” he said. “There are also interesting questions related to using this technique for realizing special nondestructive measurements that might operate at or near the standard quantum limit on quantum phase measurement.”

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...